Ultrasonic transducers which, when operating in the transmit mode, convert electrical pulses into sound waves at ultrasonic frequencies (and which, when operating in the receive mode, generate electrical signals when a sound wave at an ultrasonic frequency is incident upon them) are well known. They are used in a variety of ultrasound echo ranging applications, including in ultrasound scanning equipment used for medical diagnosis and in non-destructive testing.
The conventional ultrasonic transducer comprises a single transducer element sandwiched between front and back electrodes. The element may be a polarised ferro-electric ceramic, or a polarised polymer piezoelectric composite ceramic and/or polymer material. An alternative known form of transducer, the dual back-face electrode transducer, comprises a pair of transducer elements poled in opposite directions, with connections only to the two back electrodes of the transducer.
A problem that has existed for a considerable time is that an ultrasonic transducer is susceptible to electrical noise in the environment in which the transducer is used. This is due to the unbalanced nature of the driving circuit, and is accentuated by a high output impedance of the transducer. Even when shielded (coaxial) cables are used with the transducer, there is a significant tendency for noise pick-up. Thus considerable care (and expense) is required to maintain a high signal to noise ratio in conditions where environmental electrical noise may swamp the ultrasonic signals that are generated or received by the transducer (particularly when the transducer is operating in the receive mode).
One technique for reducing noise in transducers--particularly directed to microphone electroacoustical transducers which include elements made from barium titanate or PZT (PbZrO.sub.3.PbTiO.sub.3)--is described in the specification of U.S. Pat. No. 4,751,418. That technique involves the use of a reference voltage source (which may be earth potential) connected to the input line from the transducer to a differential amplifier. The reference voltage is established using a load having substantially the same impedance as the transducer element. The load may be a second transducer element (as shown in FIG. 1 of the specification of U.S. Pat. No. 4,751,418) or it may comprise an RC circuit (as shown in FIG. 2 of that specification).
Although the approach to noise reduction described in the specification of U.S. Pat. No. 4,751,418 may be suitable for adoption with audio frequency microphones (note that reference is made in that specification to the use of that invention in the detection of heartbeat sounds), it does not confer immunity against noise pick up in the transducer elements themselves, as these have an unbalanced configuration.